The present invention relates to adhesives and coatings and their methods of use.
When conventional, liquid, nonviscous adhesives or coatings are applied to slanted or vertical surfaces, the adhesive or coating runs. Common industrial examples of applications wherein adhesives are applied to slanted or vertical surfaces include assembly of composite structural panels, honeycomb sandwich panels, wiring brackets or the like. In many instances, the application of conventional adhesives or coatings to nonhorizontal surfaces results in potential adhesive gaps near the upper portions of the surface as well as nonuniform thickness of the coating along the surface. Conventional adhesives and coatings have been formulated that are relatively viscous fluids and are slow to run at ambient temperature; however, they become less viscous and run when heated to the necessary curing temperature. Viscous adhesives are also difficult to apply in many circumstances.
A particular illustration of the inadequacy of conventional adhesives is apparent in the airline industry where structural honeycomb cores are sandwich bonded between skins of fiber reinforced composites to form, for example, structural panels. Generally, liquid adhesive is applied to the skins as a thick coating and the skins are then placed on the faces of the honeycomb core. If the initial viscosity of the adhesive is low at ambient temperature or when heated to promote curing, the adhesive runs away from the skin-to-core joints into the cells. Adhesive gaps often result and the bond is poor.
U.S. Pat. No. 4,595,445 to Hombach et al. addresses the problem of honeycomb assembly and suggests a thermosetting polyurethane urea reactive adhesive that is stable at ambient temperature but rapidly sets at higher temperatures. An important feature of the Hombach compositions is their capacity for storage at ambient temperature and their rapid thickening at the curing temperatures, which prevents the adhesive from running. Some materials, however, cannot be exposed to the elevated temperatures that are required to thicken and cure the adhesive composition described by Hombach et al., thereby limiting its usefulness.
The present invention provides a solution to the running problem that is independent of temperature and is applicable for use with most conventional adhesives.